1. Field of the Invention
The invention relates generally to an alloy for aircraft roller bearings, as well as an alloy for a bearing or bearing part.
2. Discussion of Background Information
During operation, roller bearings of vehicles are typically subjected to diverse loads and stresses, which they are to withstand for as long as possible. These include, e.g., dynamic mechanical loads through sliding against one another or rolling of bearing parts and corrosion attack by corrosive lubricants. In the case of aircraft, an additional problem is that working temperatures of roller bearings can be in the range of several hundred degrees Celsius. For example, in the case of roller bearings of aircraft turbines temperatures of around 250° C. can be measured even in the coasting down phase when, although a load is slight, there is no longer any cooling.
Particularly high demands are therefore made on roller bearings for aircraft, e.g., with respect to load-bearing capacity and operational life in order to be serviceable. Great strength and toughness, low wear and low rolling contact fatigue in use are required as well as a high corrosion resistance even at increased temperatures. In addition, a surface of the roller bearing should have a satisfactory reactivity with respect to the additive tricresyl phosphate, which is present in the predominantly used aviation turbine oil, e.g., Mobil Jet Oil II, so that a protective reaction layer can be formed to minimize wear. The reactivity of the bearing surface with the lubricant additive depends considerably on the chemical nature of the bearing surface. Considered overall, this produces a complex profile of requirements for roller bearings for aircraft. This profile is to be met by the use of a suitable alloy.
According to DIN 17230, the most common roller bearing materials are subdivided into five groups, namely:
(1) through-hardenable roller bearing steels (e.g., 1000r6 or SAE 52100),
(2) case-hardening steels (e.g., 17MnCr5 or SAE 8620),
(3) quenched and tempered steels (e.g., 43CrMo4 or SAE 4340),
(4) corrosion-resistant steels (e.g., AISI 440C, X30CrMoN15 or X45Cr13), and
(5) heat-resistant steels and hard alloys (e.g., M50 or AISI T1).
Out of the available material groups, heat-resistant steels have become accepted for the bearings of aircraft, whereby the alloy M50, a low-alloy high-speed steel, and variants of this alloy are chiefly used. The leading role held for decades by the alloy M50 as roller bearing material for aircraft is due to its mechanical properties and good fatigue properties. However, the corrosion resistance is completely unsatisfactory, but this has been tolerated until now due to a lack of alternative alloys.
Since there is a consistent desire for more efficient and more reliable roller bearings, attempts are being made to find improved alloys comparable to alloy M50. For example, U.S. Pat No. 4,150,978, which is expressly incorporated herein by reference in its entirety, discloses individual alloys in the composition range (in % by weight) 0.8 to 1.6% carbon, max. 0.5% silicon, max. 0.5% manganese, max. 0.1% sulfur, max. 0.015% phosphorus, 12 to 20% chromium, 2 to 5% molybdenum, up to 3% tungsten, 0.5 to 3.0% vanadium, up to 0.5% titanium, max. 0.03% aluminum, max. 0.5% nickel, max. 0.5% cobalt, max. 0.5% copper, max.0.05% boron, max. 0.05% nitrogen, the balance being iron and impurities. Compared to M50, these alloys exhibit a better behavior in the rolling contact test and should also be useable in corrosive media, but have not become accepted in practice.
Another approach today lies in using surface hardening of corrosion-resistant alloys with max. 0.1% by weight carbon and chromium contents of at least 13% by weight. However, in order to obtain adequate surface hardnesses and thus an adequate wear resistance, these materials have to be subjected to case hardening methods, such as, e.g., carburization and nitridation. However, the corrosion properties can be considerably affected by the carburization or nitridation process.
Although known alloys for aircraft roller bearings can in each case conform to several properties with regard to the profile of requirements described at the outset, they fall down considerably on at least one property, e.g., corrosion resistance. Such a drop-off in one property is usually enough to reduce the operational life substantially, thus restricting the field of application for a roller bearing. It is ultimately irrelevant for the value and the useful life of a roller bearing whether it needs to be replaced due to the occurrence of fatigue or corrosion. In other words, the best mechanical properties cannot be utilized if corrosion leads to premature failure of the bearing. Conversely, the highest corrosion resistance is useless if fatigue fractures and/or premature wear occur after a short time in use.